Metal-contact-free photodetector

ABSTRACT

A Ge-on-Si photodetector constructed without doping or contacting Germanium by metal is described. Despite the simplified fabrication process, the device has responsivity of 1.24 A/W, corresponding to 99.2% quantum efficiency. Dark current is 40 nA at −4 V reverse bias. 3-dB bandwidth is 30 GHz.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of co-pending U.S.provisional patent application Ser. No. 61/950,816, filed Mar. 10, 2014,which application is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT

This invention was made with government support under GrantsFA9550-13-1-0027 and FA9550-10-1-0439 awarded by AFOSR. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The invention relates to photodetectors in general and particularly togermanium photodetectors.

BACKGROUND OF THE INVENTION

Traffic on the Internet keeps growing, due in large part to theincreasing demand from mobile devices, streaming media services, cloudcomputing, and big data analysis. Silicon photonics is promising forproviding high-speed, low energy consumption and low cost nextgeneration data communication systems. The last decade has witnesseddramatic improvement and maturity of silicon photonics devices. Highquality hybrid integrated lasers with sub-MHz linewidth, modulators andphotodetectors supporting 40 Gb/s or higher data rates have all beendemonstrated. See T. Creazzo, E. Marchena, S. B. Krasulick, P. K.-L. Yu,D. Van Orden, J. Y. Spann, C. C. Blivin, L. He, H. Cai, J. M.Dallesasse, R. J. Stone, and A. Mizrahi, “Integrated tunable CMOSlaser,” Opt. Express 21(23), 28048-28053 (2013); S. Yang, Y. Zhang, D.W. Grund, G. A. Ejzak, Y. Liu, A. Novack, D. Prather, A. E.-J. Lim,G.-Q. Lo, T. Baehr-Jones, and M. Hochberg, “A single adiabaticmicroring-based laser in 220 nm silicon-on-insulator,” Opt. Express22(1), 1172-1180 (2014); D. J. Thomson, F. Y. Gardes, J.-M. Fedeli, S.Zlatanovic, Y. Hu, B. P.-P. Kuo, E. Myslivets, N. Alic, S. Radic, G. Z.Mashanovich, and G. T. Reed, “50-Gb/s silicon optical modulator,” IEEEPhoton. Technol. Lett. 24(4), 234-236 (2012); T. Baba, S. Akiyama, M.Imai, N. Hirayama, H. Takahashi, Y. Noguchi, T. Horikawa, and T. Usuki,“50-Gb/s ring-resonator-based silicon modulator,” Opt. Express 21(10),11869-11876 (2013); C. T. DeRose, D. C. Trotter, W. A. Zortman, A. L.Starbuck, M. Fisher, M. R. Watts, and P. S. Davids, “Ultra compact 45GHz CMOS compatible Germanium waveguide photodiode with low darkcurrent,” Opt. Express 19(25), 24897-24904 (2011); and L. Vivien, A.Polzer, D. Marris-Morini, J. Osmond, J. M. Hartmann, P. Crozat, E.Cassan, C. Kopp, H. Zimmermann, and J. M. Fédéli, “Zero-bias 40 Gbit/sgermanium waveguide photodetector on silicon,” Opt. Express 20(2),1096-1101 (2012).

Transceivers and switch fabrics monolithically integrated withelectronics have been reported. See B. Analui, D. Guckenberger, D.Kucharski, and A. Narasimha, “A fully integrated 20-Gb/s optoelectronictransceiver implemented in a standard 0.13-μm CMOS SOI technology,” IEEEJ. Solid-State Circuits 41(12), 2945-2955 (2006); J. F. Buckwalter, X.Zheng, G. Li, K. Raj, and A. V. Krishnamoorthy, “A monolithic 25-Gb/stransceiver with photonic ring modulators and Ge detectors in a 130-nmCMOS SOI process,” IEEE J. Solid-State Circuits 47(6), 1309-1322 (2012);and B. G. Lee, A. V. Rylyakov, W. M. J. Green, S. Assefa, C. W. Baks, R.Rimolo-Donadio, D. M. Kuchta, M. H. Khater, T. Barwicz, C. Reinholm, E.Kiewra, S. M. Shank, C. L. Schow, and Y. A. Vlasov, “Monolithic siliconintegration of scaled photonic switch fabrics, CMOS logic, and devicedriver circuits,” J. Lightw. Technol. 32(4), 743-751 (2014). Coherentlong-haul communication at 112 Gb/s was also demonstrated. See P. Dong,X. Liu, S. Chandrasekhar, L. L. Buhl, R. Aroca, Y. Baeyens, and Y.-K.Chen, “Monolithic silicon photonic integrated circuits for compact100+Gb/s coherent optical receivers and transmitters,” IEEE J. Sel.Topics Quantum Electron. 20(4), 6100108 (2014). Foundry services openaccess of advanced fabrication nodes to academic labs and startups,which would further speed up research and development of photonicintegration on silicon. Se for example M. Hochberg and T. Baehr-Jones,“Towards fabless silicon photonics,” Nat. Photonics 4, 492-494 (2010);and A. E.-J. Lim, J. Song, Q. Fang, C. Li, X. Tu, N. Duan, K. K. Chen,R. P.-C. Tern, and T.-Y. Liow, “Review of silicon photonics foundryefforts,” IEEE J. Sel. Topics Quantum Electron. 20(4), 8300112 (2011).

One bottleneck that emerges during the design of silicon photonics baseddata links is the constraint on link power budget. A typical link powerbudget is around 9 dB. For example the IEEE 802.3 40GBASE-LR4 protocolhas 6.7 dB allocated for channel insertion loss and 2.3 dB forpenalties. Due to the large mode mismatch of glass fibers and submicronsilicon waveguides, on-and-off chip coupling loss is usually quite high.The losses can exceed 1 dB in a mature commercial process. See A. Mekis,S. Gloeckner, G. Masini, A. Narasimha, T. Pinguet, S. Sahni, and P. DeDobbelaere, “A grating-coupler-enabled CMOS photonics platform,” IEEE J.Sel. Topics Quantum Electron. 17(3), 597-608 (2011). On-chip devicestend to be lossy as well. For example, insertion losses of state of theart silicon modulators are more than 5 dB. In some cases, deviceinsertion loss could be significantly reduced by design optimization,such as the y-junction, the waveguide crossing and by grating couplers.See Y. Zhang, S. Yang, A. E.-J. Lim, G.-Q. Lo, C. Galland, T.Baehr-Jones, and M. Hochberg, “A compact and low loss Y-junction forsubmicron silicon waveguide,” Opt. Express 21(1), 1310-1316 (2013); Y.Ma, Y. Zhang, S. Yang, A. Novack, R. Ding, A. E.-J. Lim, G.-Q. Lo, T.Baehr-Jones, and M. Hochberg, “Ultralow loss single layer submicronsilicon waveguide crossing for SOI optical interconnect,” Opt. Express21(24), 29374-29382 (2013); and W. S. Zaoui, A. Kunze, W. Vogel, M.Berroth, J. Butschke, F. Letzkus, and J. Burghartz, “Bridging the gapbetween optical fibers and silicon photonic integrated circuits,” Opt.Express 22(2), 1277-1286 (2014). However, in other cases, insertion lossand device efficiency are orthogonal, for example, higher doping resultsin higher modulation efficiency, but leads to high insertion loss at thesame time.

A photodetector with high responsivity will compensate the channelinsertion loss, and help fulfill the required link power budget.Germanium can be epitaxially grown on silicon and is the preferredabsorber material for its CMOS compatibility. Althoughmetal-semiconductor-metal (MSM) and avalanche photodetector (APD) couldprovide high responsivity by photoconductive gain and avalanchemultiplication, the benefit comes at the price of high dark current and(or) high bias voltage. Waveguide coupled p-i-n detectors attractextensive attention due to their high bandwidth, good responsivity andlow dark current. Ge-on-Si detectors with lateral and vertical p-i-njunction configuration are illustrated in FIG. 1A and FIG. 1B.Attractive Ge-on-Si detector performances have been reported, withresponsivity typically about 0.8 A/W and bandwidth high enough for 40Gb/s operation. See for example T. Yin, R. Cohen, M. M. Morse, G. Sarid,Y. Chetrit, D. Rubin, and M. J. Paniccia, “31 GHz Ge n-i-p waveguidephotodetectors on silicon-on-insulator substrate,” Opt. Express 15(21),13965-13971 (2007); and A. Novack, M. Gould, Y. Yang, Z. Xuan, M.Streshinsky, Y. Liu, G. Capellini, A. E.-J. Lim, G.-Q. Lo, T.Baehr-Jones, and M. Hochberg, “Germanium photodetector with 60 GHzbandwidth using inductive gain peaking,” Opt. Express 21(23),28387-28393 (2013) as well as some of the previously mentioned articles.

As shown in FIG. 1A and FIG. 1B, both types of device require heavilydoped germanium to form the junction and direct contact of germanium tometal via plugs. Although the first transistor was demonstrated usinggermanium, silicon quickly took over and became the overwhelminglydominating substrate material. Germanium processing has recentlyattracted attention because of interest in germanium andsilicon-germanium transistors. See S. Brotzmann, and H. Bracht,“Intrinsic and extrinsic diffusion of phosphorus, arsenic, and antimonyin germanium,” J. Appl. Phys. 103, 033508 (2008), A. Claverie, S.Koffel, N. Cherkashin, G. Benassayag, and P. Scheiblin, “Amorphization,recrystallization and end of range defects in germanium,” Thin SolidFilms 518(9), 2307-2313 (2010); and H. Bracht, S. Schneider, and R.Kube, “Diffusion and doping issues in germanium,” Microelectron. Eng.88(4), 452-457 (2011). Germanium is much less well understood andcharacterized as compared to silicon. While silicon modulators have beenoptimized for efficiency (see Y. Liu, S. Dunham, T. Baehr-Jones, A.E.-J. Lim, G.-Q. Lo, and M. Hochberg, “Ultra-responsive phase shiftersfor depletion mode silicon modulators,” J. Lightwave Technol. 31(23),3787-3793 (2013)), similar TCAD models are still not seen for germaniumdetectors. Poly silicon was sometimes deposited on top of germanium toreduce contact resistivity and leakage current. See for example, C.-K.Tseng, W.-T. Chen, K.-H. Chen, H.-D. Liu, Y. Kang, N. Na, and M.-C. M.Lee, “A self-assembled microbonded germanium/silicon heterojunctionphotodiode for 25 Gb/s high-speed optical interconnects,” Sci. Rep. 3,3225 (2013); and K. Takeda, T. Hiraki, T. Tsuchizawa, H. Nishi, R. Kou,H. Fukuda, T. Yamamoto, Y. Ishikawa, K. Wada, and K. Yamada,“Contributions of Franz-Keldysh and avalanche effects to responsivity ofa germanium waveguide photodiode in the L-band,” IEEE J. Sel. TopicsQuantum Electron. 20(4), 3800507 (2014).

There is a need for improved designs and structures for photodetectorsmade using germanium.

SUMMARY OF THE INVENTION

According to one aspect, the invention features a germanium p-i-nphotodetector having a floating germanium body. The detector is alsoreferred to as a metal-contact-free photodetector.

The germanium photodetector comprises a first doped semiconductorcontact; a second doped semiconductor contact; and an intrinsicgermanium body in electrical contact with the first doped semiconductorcontact and in electrical contact with the second doped semiconductorcontact, the first doped semiconductor contact and the second dopedsemiconductor contact disposed on a same side of the intrinsic germaniumbody, the intrinsic germanium body lacking direct mechanical contactwith a metal contact; the first doped semiconductor contact and thesecond doped semiconductor contact in electrical communication withrespective metal terminals configured to provide electrical signalsgenerated in the germanium photodetector by absorption ofelectromagnetic radiation to circuitry external to the germaniumphotodetector.

In one embodiment, at least one of the first doped semiconductor contactand the second doped semiconductor contact is a doped silicon contact.

In one embodiment, at least one of the first doped semiconductor contactand the second doped semiconductor contact is doped with a p-typedopant.

In one embodiment, at least one of the first doped semiconductor contactand the second doped semiconductor contact is doped with an n-typedopant.

In one embodiment, the germanium photodetector further comprises a thirddoped semiconductor contact.

In one embodiment, wherein the intrinsic germanium body has a triangularcross section.

In one embodiment, the intrinsic germanium body is terminated in a (111)crystallographic face.

In still another embodiment, the intrinsic germanium body isplanaraized.

In another embodiment, a crystallographic facet is oriented at an anglebetween substantially 15 degrees and 75 degrees to the surface of thesilicon wafer.

In another embodiment, a measured quantum efficiency is greater thansubstantially 65%.

In yet another embodiment, a measured quantum efficiency is greater thansubstantially 75%.

In still another embodiment, a measured quantum efficiency is greaterthan substantially 85%.

In a further embodiment, a measured quantum efficiency is greater thansubstantially 95%.

In yet another embodiment, the electromagnetic radiation is in thewavelength range of substantially 1280-1600 nm in free space.

In another embodiment, the photodetector includes a p-i-n junction.

According to another aspect, the invention relates to a method ofdetecting electromagnetic radiation with a germanium photodetector. Themethod comprises providing a germanium photodetector, comprising: afirst doped semiconductor contact; a second doped semiconductor contact;and an intrinsic germanium body in electrical contact with the firstdoped semiconductor contact and in electrical contact with the seconddoped semiconductor contact, the first doped semiconductor contact andthe second doped semiconductor contact disposed on a same side of theintrinsic germanium body, the intrinsic germanium body lacking directmechanical contact with a metal contact; the first doped semiconductorcontact and the second doped semiconductor contact in electricalcommunication with respective metal terminals configured to provideelectrical signals generated in the germanium photodetector byabsorption of electromagnetic radiation to circuitry external to thegermanium photodetector; receiving electromagnetic radiation by thegermanium photodetector; generating electrical signals representative ofan intensity of the electromagnetic radiation or representative of dataencoded in the electromagnetic radiation; and performing at least one ofdisplaying the electrical signals to a user and recording the electricalsignals in a machine-readable memory in non-volatile form.

In one embodiment, the germanium photodetector supports single opticalmode operation.

In another embodiment, the electromagnetic radiation is in thewavelength range of substantially 1280-1600 nm in free space.

In still another embodiment, the germanium photodetector supportsconduction mode operation.

In yet another embodiment, the germanium photodetector supportsoperation in avalanche photodiode mode.

In still a further aspect, the invention provides a method offabricating a germanium photodetector. The method comprises patterning adoped semiconductor wafer having a surface by lithography and etching tocreate waveguides on the surface; doping the doped semiconductor byimplantation and annealing to form a p-type contact and an n-typecontact in a layer at the surface of the doped semiconductor wafer;performing germanium epitaxy to provide an intrinsic germanium bodyhaving no deliberately added dopant in contact with the n-type contactand the p-type contact, the germanium body having a shape bounded bycrystallographic facets oriented at an angle to the surface of the dopedsemiconductor wafer; and applying metallization to form contactterminals.

In one embodiment, the photodetector is sensitive to electromagneticradiation is in the wavelength range of substantially 1280-1600 nm infree space.

In another embodiment, the photodetector includes a p-i-n junction.

In a further embodiment, a crystallographic facet is oriented at anangle between substantially 15 degrees and 75 degrees to the surface ofthe silicon wafer.

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent from the following descriptionand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout the various views.

FIG. 1A is a schematic cross-section diagram of a prior art lateralp-i-n Ge-on-Si photodetector.

FIG. 1B is a schematic cross-section diagram of a prior art verticalp-i-n Ge-on-Si photodetector.

FIG. 2A is a schematic cross-section diagram of an embodiment of afloating germanium detector according to principles of the invention.

FIG. 2B is a scanning electron micrograph of the intrinsic germaniumbody and the surrounding structure in another embodiment of a fabricatedfloating germanium detector.

FIG. 3A is a graph of an optical mode profile of the floating germaniumdetector of FIG. 2A at −4V reverse bias.

FIG. 3B is a graph of the electrical field of the floating germaniumdetector of FIG. 2A at −4V reverse bias.

FIG. 4 is a graph showing transmission spectra of floating germaniumdetectors and a reference grating coupler (GC).

FIG. 5A is a graph of a device IV curve in the dark.

FIG. 5B is a graph of a device IV curve with a laser on.

FIG. 5C is a graph of a device responsivity as a function of reversebias voltage.

FIG. 6 is a graph of a device S21 at different reverse bias voltages vsfrequency.

FIG. 7A through FIG. 7D show cross sections of a wafer as it is beingfabricated into a device that embodies aspects of the invention.

DETAILED DESCRIPTION

We describe a novel floating germanium photodetector that significantlysimplifies Ge-on-Si detector fabrication process by eliminating the needto dope and contact germanium. The epitaxial Ge is not deliberatelydoped. It keeps germanium intact from damage and preserves the crystalquality after epitaxy. The device was measured to have responsivity of1.24 A/W at 1550 nm wavelength, corresponding to 99.2% quantumefficiency. To the best of our knowledge, this is the highestresponsivity reported for p-i-n germanium detectors. At −4V reversebias, dark current is only 40 nA. The measured 3-dB bandwidth is 30 GHzand capacitance is 8 fF. The detector functions for optical radiationwith free-space wavelengths from 1280-1600 nm.

The detector geometry allows the optical radiation to be is coupledpredominantly into a single mode within the combined Ge/silicon detectorgeometry, thus maximizing the chance for absorption. It is believed thatthe optical mode is prevented from leaking into the silicon contacts dueto the high index of refraction of Ge.

Material Refractive Index Si 3.48 Ge 4.01

The detector geometry is useful to avoid the requirement of contactingmetal or a conductive alloy (such as Al or TaN) directly to the Ge, thussimplifying fabrication processes. It is believed that detectorperformance is improved because metal is not in close proximity with theoptical mode.

The detector geometry is useful to avoid the requirement of implantingGe with dopant species, thus improving optical performance by virtue ofreduced absorption by impurities. The elimination of doping of the Gealso simplifies fabrication and reduces cost.

Device Design

A schematic illustration of the floating germanium photodetector isshown in FIG. 2A. As illustrated in FIG. 2A, a silicon wafer, such as asilicon on insulator (SOI) wafer is employed. A buried oxide (BOX) layeris present on the silicon wafer that serves as a handle. The BOX layerinsulates the silicon on which the device is fabricated from the siliconhandle. In FIG. 2A the intrinsic germanium body is shown as having atriangular cross section. The triangular shape of the germanium in FIG.2A has been realized in actual device fabrication using chemical vapordeposition (CVD). For fabrication, both plasma enhanced (PE) CVD andultra-high vacuum (UHV) CVD have been employed. In general, an intrinsicsemiconductor, also called an undoped semiconductoror i-typesemiconductor, is a pure semiconductor without any significant dopantspecies present. The number of charge carriers is therefore determinedby the properties of the material itself instead of the amount ofimpurities. In particular, an intrinsic semiconductor, such as intrinsicgermanium, is generally understood to have few or no deliberately addeddopants, although it is understood that some amount of dopants orimpurities may well be present.

The device illustrated in FIG. 2A has silicon n-type and p-type contactsthat are in electrical communication with the opposite ends of agermanium triangular body. The silicon n-type and p-type contacts arerespectively in electrical communication with metal terminals that serveto provide electrical signals generated in the photodetector tocircuitry external to the photodetector.

FIG. 2B is a scanning electron micrograph of the intrinsic germaniumbody and the surrounding structure in another embodiment of a fabricatedfloating germanium detector. FIG. 2B shows an alternative shape of theintrinsic germanium body, in which the intrinsic germanium body has across section that is defined by a number of facets that togetherprovide a non-planar faceted shape. The shape is not a parallelepipedcross section, nor a triangular cross section, nor a truncated prismcross section as illustrated in FIG. 1B. The shape illustrated in FIG.2B is fabricated using PECVD or UHVCVD with different depositionconditions from those used to produce the triangular shape illustratedin FIG. 2A.

In some embodiments, the germanium body may be planarized, for exampleby being subjected to a mechanical or chemical-mechanical polishing(CMP) process. In some embodiments, the planarized germanium body mayhave a third electrical contact in electrical communication with theplanar surface produced in the planarization process.

It is expected that in some embodiments, the germanium photodetectordevice can be operated as a conduction device, in which the conductiveproperties of the germanium are changed under illumination.

It is expected that in some embodiments, the germanium photodetector canbe operated as an avalanche photodetector (APD). It is expected thatphotomultiplication can occur either in the germanium or in the silicon.

It is contemplated that in some embodiments, an external heater (such asa resistive heater) can be provided to keep the detector at an elevatedtemperature for improved performance.

Compared to conventional detector configuration in FIG. 1A and FIG. 1B,germanium is protected from defects caused by ion implantation damageand metallization. Note that creating metal via plugs is a complicatedprocess that requires contact hole opening, silicide formation,diffusion barrier deposition, and finally metal deposition, patterningand planarization, in addition to the implantation and annealing steps.The process is described in J. D. Plummer, M. Deal, P. D. Griffin,“Silicon VLSI technology: fundamentals, practice, and modeling,”(Prentice Hall, 2000). The floating germanium detector configurationsignificantly simplifies the silicon photonics process flow and reducescost for building silicon based photonics integrated circuits (PICs).Since it shares exactly the same doping levels and metallizationprocedures of a silicon modulator, germanium epitaxy is the only extrastep to build the device in addition to those present in a process toconstruct a modulator.

We now discuss the triangular shape of the germanium illustrated in FIG.2A. The growth rate of crystalline germanium is different in differentdirections, which is analogous to the anisotropic wet etch of silicon,which naturally stops at the (111) surface due to a much slower etchrate. Germanium geometry also depends on the trench angle of the oxidehard mask, and could be projected by the Wulff construction model. SeeJ. Liu, R. Camacho-Aguilera, J. T. Bessette, X. Sun, X. Wang, Y. Cai, L.C. Kimerling, and J. Michel, “Ge-on-Si optoelectronics,” Thin SolidFilms 520(8), 3354-3360 (2012). In one embodiment, the epitaxialgermanium was measured to have a 25° sidewall angle versus the siliconsurface in the process to build our device. With a germanium base widthof 1.5 μm, the triangle height is 0.35 μm.

Despite the simplified fabrication, the floating germanium detector isexpected to have higher responsivity than conventional germaniumdetectors as shown in FIG. 1A and FIG. 1B, because absorption by metalatoms or ions introduced from metal electrodes and free carrierabsorption from heavy contact doping are eliminated. Dark current isalso expected to be lower because of the preserved crystal quality afterepitaxy. To achieve high responsivity, photons should be confined in theintrinsic germanium absorber, and scattering needs to be minimized. Thefundamental mode of the germanium silicon hybrid waveguide structure ofFIG. 2A is plotted in FIG. 3A. Tight mode confinement in germanium, withconfinement factor 88%, ensures efficient absorption and minimizesdetector length, and thus capacitance. A 3 μm long germanium taper from0.22 μm to 1.5 μm in width is used to adiabatically convert light fromthe input silicon waveguide to the hybrid waveguide.

In absence of the p-i-n junction formed in germanium, the device relieson the fringe field of the silicon junction to sweep out photo-generatedcarriers. It has been reported that the fringe field and thecorresponding capacitance is a non-negligible part of the 220 nm thicksilicon pn junction and needs to be accounted for in modulator design.See H. J. Wesley, D. Sacher, and J. K. S. Poon, “Analytical model andfringing-field parasitics of carrier-depletion Silicon-on-Insulatoroptical modulation diodes,” IEEE Photon. J. 5(1), 2200211 (2013). Asgermanium has a much higher permittivity than typical CMOS dielectrics,such as silicon nitride or silicon dioxide, the portion of fringe fieldand capacitance will be even higher for the same silicon junction. Thejunction intrinsic region width in FIG. 3B is selected to match the modefield diameter in FIG. 3A. We numerically solved Poisson's equation andplotted the electrical field distribution in FIG. 3B. The electric fieldin most parts of the germanium is greater than 10⁴ V/cm, which is highenough for the carriers to drift at saturation velocity. See C.Jacoboni, F. Nava, C. Canali, and G. Ottaviani, “Electron drift velocityand diffusivity in germanium,” Phys. Rev. B 24(2), 1014-1026 (1981).

Device Fabrication

We produced a prototype of the device. The floating germanium detectorwas fabricated using the standard process to create conventional p-i-ndetectors with 0.5 μm thick germanium slab, and no additional processsplit was added thanks to the anisotropic epitaxial growth of germanium.The starting substrate was an 8-inch silicon on insulator (SOI) wafer,with 220 nm, 10 ohm-cm p-type top silicon film, and 2 μm buried oxide ontop of a high resistivity silicon handle. Waveguides and gratingcouplers were patterned using 248 nm UV lithography followed by dryetching. Boron and phosphorus ions were then implanted into silicon, andactivated by rapid thermal annealing. Germanium epitaxy followed. Twolayers of aluminum metal interconnect completed the fabrication process.

FIG. 7A through FIG. 7D show cross sections of a wafer as it is beingfabricated into a device that embodies aspects of the invention. FIG. 7Aillustrates the steps of patterning silicon by lithography and etchingto create waveguides. FIG. 7B illustrates the steps involved in dopingthe silicon by implantation and annealing to form a p-type contact andan n-type contact. FIG. 7C illustrates the step of performing germaniumepitaxy. FIG. 7D illustrates the steps of applying metallization toprovide contact terminals. As needed, vias may be defined in layers ofthe structure to allow the contact terminals to extend therethrough soas to be accessible at a free surface of the completed structure. Somesteps need to be performed in a specific sequence, and some steps mightbe performed in alternative sequences (or in any order). For example,boron and phosphorus ion implantation can be done in either order.

Device Characterization Optical Spectrum

Two sets of characterization structures corresponding to the devicecross-section in FIG. 2A were designed. Grating couplers were used asoptical I/O to a fiber array in both cases. In Set A, transmitted lightafter the germanium absorber was guided to another grating coupler,which was used to characterize the germanium efficiency and determinethe device length. In Set B, the through port was connected to ay-junction with its two branches tied together, which effectivelyfunctioned as a broadband mirror. A fiber array was first aligned to thegrating couplers in Set A, and the devices ware measured using a tunablelaser (Agilent 81600B). The spectra of two devices with differentgermanium length, as well as a reference grating coupler, are plotted inFIG. 4. The parabolic line shape was determined by the grating couplerspectral response. A reduction in power level indicates extra loss addedby the germanium strip. No interference fringes were observed on thespectrum, confirming that light stayed in its fundamental modethroughout the structure. Single mode operation prevented the loss orwaste of photons from scattering or divergence, and also improvedabsorption per unit length since light was tightly confined in thegermanium absorber. The capability to couple light upward into germaniumand back down into silicon is useful for constructing germaniumabsorption modulators as well. The lengths of the two detectors measuredas shown in FIG. 4 were 11 μm and 16 μm respectively, including 6 μm fortapers. Stronger absorption towards shorter wavelength is clearlyillustrated, because shorter wavelength is further from the band edge ofgermanium. At 1550 nm, the 16 μm long germanium caused 26 dBattenuation. With the y-junction loop mirror to reflect the transmittedphotons back for reabsorption, the 16 μm long detector in device Set Bwill be able to achieve almost 100% quantum efficiency, if allphoto-generated carriers are effectively collected by the electrodes.

IV Sweeps

In addition to the optical properties, device performance also dependson the p-i-n junction shown in FIG. 3B. We probed the device andcharacterized the IV curve using a semiconductor device analyzer(Agilent B1500A), both in the dark and with a laser on, as shown in FIG.5A and FIG. 5B. In FIG. 5A, the dark current is only 40 nA at −4V andstays below 90 nA up to reverse bias of −8 V, which is an order ofmagnitude smaller than the dark current of conventional vertical p-i-ndetectors fabricated in the same process. We attribute this improvementto the smaller junction area and preserved germanium crystal qualityafter epitaxy. When the laser was turned on and set to 1550 nm, lightimpinged on the detector input port was 0.48 mW after normalizing outthe grating coupler insertion loss. The photocurrent increases as thereverse bias voltage and saturates at about −2 V. Below −2 V, the fringefield is not strong enough to sweep out photo-generated carriers beforethey recombine. Above −2 V, all photo-generated carriers are swept outwithin their lifetime and are collected by the electrodes. Hence thephotocurrent saturates and stays relatively flat until beyond −6 V,where it slightly tails up due to the onset of avalanche gain.

Responsivity as a function of bias voltage, extracted from the IV curveunder illumination, is plotted as FIG. 5C. The responsivity is 1.24 A/Wat −4 V, corresponding to 99.2% quantum efficiency, which is asignificant improvement compared to 0.75 A/W achieved in conventionaldetectors fabricated in the same process. At 1550 nm wavelength, thetheoretical maximum responsivity a detector could provide is given by

$R = {\frac{I_{ph}}{P_{inc}} = {\frac{e}{hv} = {1.25\mspace{14mu} A\text{/}W}}}$

where e is the electron charge, h is Planck's constant and v is theoptical frequency. To the best of our knowledge, this is the highestresponsivity or quantum efficiency reported for Ge-on-Si p-i-nphotodetectors, without photoconductive or avalanche gain. Highresponsivity or quantum efficiency of this device is attributed to acombination of multiple factors, including the preserved crystalquality, elimination of metal and free carrier absorption that do notgenerate photocurrent, minimization of scattering and divergence bysingle mode guiding, and effective collection of photo-generatedcarriers using the junction fringe field.

Bandwidth and Capacitance

Excellent performance was achieved at DC. We next discuss the devicebandwidth and capacitance. Photocurrent roll off was characterized bymeasuring the s-parameters using a VNA (Agilent E8361C) and a LiNO₃modulator (Thorlab LN05S). S21 traces at different bias voltages wereplotted in FIG. 6. At −2 V the carriers travel at relatively low speedeven though the fringe field is strong enough to sweep out thephoto-generated carriers, which limits the device bandwidth to onlyabove 5 GHz. As −4 V and −6 V, carriers approach their saturationvelocity, and device bandwidth increases to above 30 GHz, which issufficient to support 40 Gb/s data rates.

Generally photodetector bandwidth is determined either by carriertransit time or device RC time constant. Taking the saturation velocityto be 6.5×10⁶ cm/s, and the mode field diameter to be 0.85 μm, thetransit time is estimated to be

$f_{t} = {\frac{0.44v_{sat}}{L} = {33.6\mspace{14mu} {GHz}}}$

which is close to the measured bandwidth. Transit time limited bandwidthcould be improved by using narrower germanium strip, which won't degradedetector efficiency given the strong absorption of germanium, as shownin FIG. 4.

Device capacitance was determined to be 8 fF, calculated from the phaseinformation of the s-parameter, which compares favorably tostate-of-the-art. Low capacitance is advantageous for the device to beused in optical interconnects to silicon chips. Assuming 50Ω loadimpedance, the major contributor of series resistance is the p+ and n+doped 90 nm silicon slab connecting the silicon underneath germanium andthe metal via. The sheet resistance at this intermediate doping level is3750Ω/□ (Ohms per square) and 1490Ω/□ for p+ and n+ silicon slabrespectively. They are 1.5 μm wide and 16 μm long, leading to around490Ω series resistance. Thus the RC time limited bandwidth is given by

$f_{RC} = {\frac{1}{2{{\pi C}_{pd}( {R_{pd} + R_{L}} )}} = {36.8\mspace{14mu} {{GHz}.}}}$

Since the light is tightly confined in germanium, it is safe to usehigher doping on these connecting slabs without introducing noticeableoptical loss from free carrier absorption. The sheet resistance for p++and n++ dope slab is 137Ω/□ and 60Ω/□, which is an order of magnitudesmaller than those of p+ and n+ slab, and will totally remove RC timelimit on device operating bandwidth.

Definitions

Unless otherwise explicitly recited herein, any reference to anelectronic signal or an electromagnetic signal (or their equivalents) isto be understood as referring to a non-volatile electronic signal or anon-volatile electromagnetic signal.

Unless otherwise explicitly recited herein, any reference to “record” or“recording” is understood to refer to a non-volatile or non-transitoryrecord or a non-volatile or non-transitory recording.

Recording the results from an operation or data acquisition, forexample, recording results such as an electrical signal having aparticular frequency or wavelength, or recording an image or a portionthereof, is understood to mean and is defined herein as writing outputdata in a non-volatile or non-transitory manner to a storage element, toa machine-readable storage medium, or to a storage device. Non-volatileor non-transitory machine-readable storage media that can be used in theinvention include electronic, magnetic and/or optical storage media,such as magnetic floppy disks and hard disks; a DVD drive, a CD drivethat in some embodiments can employ DVD disks, any of CD-ROM disks(i.e., read-only optical storage disks), CD-R disks (i.e., write-once,read-many optical storage disks), and CD-RW disks (i.e., rewriteableoptical storage disks); and electronic storage media, such as RAM, ROM,EPROM, Compact Flash cards, PCMCIA cards, or alternatively SD or SDIOmemory; and the electronic components (e.g., floppy disk drive, DVDdrive, CD/CD-R/CD-RW drive, or Compact Flash/PCMCIA/SD adapter) thataccommodate and read from and/or write to the storage media.

THEORETICAL DISCUSSION

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different from the theory presented herein will notdetract from the inventions described herein.

Any patent, patent application, patent application publication, journalarticle, book, published paper, or other publicly available materialidentified in the specification is hereby incorporated by referenceherein in its entirety. Any material, or portion thereof, that is saidto be incorporated by reference herein, but which conflicts withexisting definitions, statements, or other disclosure materialexplicitly set forth herein is only incorporated to the extent that noconflict arises between that incorporated material and the presentdisclosure material. In the event of a conflict, the conflict is to beresolved in favor of the present disclosure as the preferred disclosure.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

1-24. (canceled)
 25. A photodetector, comprising: a substrate; a devicelayer, including a waveguide, on a surface of the substrate; a firstdoped semiconductor contact in the device layer; a second dopedsemiconductor contact in the device layer; a semiconductor body, inelectrical contact with the first doped semiconductor contact and thesecond doped semiconductor contact, the semiconductor body capable ofgenerating electrical signals by absorbing electromagnetic radiation; afirst metal terminal, in electrical communication with said first dopedsemiconductor contact; and a second metal terminal, in electricalcommunication with said second doped semiconductor contact; wherein thefirst and second metal terminals are configured to provide theelectrical signals to external circuitry; wherein the first dopedsemiconductor contact comprising a first portion underneath thesemiconductor body, a second portion underneath the first metalterminal, and a connecting slab extending in the device layer betweenthe first and second portions; and wherein the second dopedsemiconductor contact comprising a first portion underneath thesemiconductor body, a second portion underneath the second metalterminal, and a connecting slab extending in the device layer betweenthe first and second portions.
 26. The photodetector according to claim25, wherein the connecting slab of at least one of the first and seconddoped semiconductor contacts comprises a higher doping level than therespective first portion.
 27. The photodetector according to claim 25,wherein the connecting slab of at least one of the first and seconddoped semiconductor contacts comprises a doping level intermediate therespective first and second portions.
 28. The photodetector according toclaim 25, wherein sheet resistance of the second portion of at least oneof the first and second doped semiconductor contacts is an order ofmagnitude smaller than that of the respective connecting slab.
 29. Thephotodetector according to claim 25, wherein at least one of said firstdoped semiconductor contact and said second doped semiconductor contactcomprises a doped silicon contact.
 30. The photodetector according toclaim 25, wherein the semiconductor body comprises germanium.
 31. Thephotodetector according to claim 25, wherein the semiconductor bodycomprises intrinsic germanium.
 32. The photodetector according to claim25, wherein said first doped semiconductor contact comprises a p-typecontact; and wherein said second doped semiconductor contact comprisesan n-type contact.
 33. The photodetector according to claim 25, whereinsaid semiconductor body comprises a plurality of facets providing anon-planar faceted shape.
 34. The photodetector according to claim 33,wherein said semiconductor body comprises a triangular cross section.35. The photodetector according to claim 34, wherein one of the facetsis oriented at an angle between 15° and 75° to an upper surface of thesubstrate.
 36. A method of fabricating a semiconductor photodetector,comprising: patterning an device layer of a semiconductor wafer bylithography; etching the device layer to create waveguide portions andcontact portions on a substrate; doping the contact portions byimplantation; annealing the contact portions to form a p-type contactand an n-type contact, each contact comprising first and secondportions, and a connecting slab extending between the first and secondportions; performing epitaxial deposition to provide a semiconductorbody in contact with the first portion of the n-type contact and thefirst portion of the p-type contact; and applying metallization to formfirst and second contact terminals in electrical communication with thesecond portion of the p-type contact and the second portion of then-type contact, respectively, but lacking direct contact with thesemiconductor body.
 37. The method according to claim 36, wherein thedoping step comprising doping the connecting slab with a higher dopinglevel than the first portion.
 38. The method according to claim 36,wherein the doping step comprises doping the connecting slab to a levelintermediate the first and second portions.
 39. The method according toclaim 36, wherein the semiconductor body comprises intrinsic germanium.40. The method according to claim 36, wherein said epitaxial depositionstep comprises forming a plurality of facets providing a non-planarfaceted shape.
 41. The method according to claim 40, wherein theepitaxial deposition step includes mechanically or chemical-mechanicalpolishing the semiconductor body.
 42. The method according to claim 40,wherein the non-planar faceted shape comprises a triangular crosssection.
 43. The method according to claim 42, wherein said epitaxialdeposition step comprises forming one of the facets to be oriented at anangle between 15° and 75° to an upper surface of the substrate.